A common way to apply coatings to moving surfaces is to create a free-falling curtain of coating composition and to pass the surface to be coated through the curtain. FIG. 1 a, b and c show examples of curtain coating means of the prior art. In some applications, such as the painting of objects, the uniformity of the coating is not critical. In such cases, simple and inexpensive means known in the art can be used to form the curtain. For example, a weir can be used. The weir can be a basin having a relatively low edge along which the coating composition overflows to form a curtain. The coating composition is pumped or poured into the basin. If the overflowing edge is horizontal, and if the basin is wide and deep, a uniform curtain is created.
However, a large basin can have disadvantages. The size required to ensure a uniform curtain may exceed available space. A large basin may require thick walls or a sturdy platform to prevent mechanical sagging due to weight. In some operations, it may not be practical to recover the coating composition remaining in the basin at the completion of a production run; in this case, the larger the volume of the basin, the greater the loss of possibly expensive coating composition. A large basin also has regions where the coating composition is nearly stagnant. Gravitationally induced inhomogeneities of the coating composition can occur in stagnant regions if the coating composition is a dispersion or suspension of one phase in another of a different density; examples include silver grains in aqueous gelatin and matte particles in a liquid. A large basin is also subject to eddies or regions of flow recirculation. Stagnation zones where recirculations meet are particularly susceptible to gravitationally induced inhomogeneities. Inhomogeneities in the coating composition can produce visual or functional nonuniformities in the coating such as streaks.
However, reducing the volume of the basin while maintaining widthwise uniformity entails difficulties. As the cross section of the basin becomes smaller, the hydrodynamic resistance to flow in the direction of the width of the curtain increases; gravitational leveling may become incomplete, and where the level is higher, the curtain will have a higher flow rate. Similarly, the disturbing effects of the incoming liquid may not completely dissipate before the curtain forms in a basin of small cross section. One such disturbance is the jetting of the coating composition into the basin when it is introduced through a conduit or poured from a nozzle.
The following U.S. Pat. Nos. describe coating apparatus useful in curtain coating: 2,745,419; 3,067,060; 3,074,374; 3,205,089; 3,345,972; 3,365,325; 3,632,374; 3,717,121; 3,876,465; 4,060,649; 4,075,976; 4,230,743; 4,384,015; 4,427,722; 5,298,288.
Ways are known in the art to preserve flow uniformity while reducing the cross section of the distributor. One way, shown in FIG. 1a, is to feed the basin from a conduit with numerous holes spanning the coating width; however, the multiple, discrete streams promote areas of recirculation and stagnation in the basin. Another way, shown in FIG. 1b, is to employ an extrusion die having a distribution cavity and narrow slot through which the liquid is forced. If the flow resistance over the length of the slot is large compared to that over the length of the cavity, the liquid is distributed across the die. In the most demanding applications, two or more cavity/slot combinations are employed in series. The effectiveness of the die also depends on the rheological properties of the coating composition. The narrow slot of the die subjects a non-Newtonian coating composition to high rates of shearing where pseudoplasticity or viscoelasticity become complicating performance factors. So, a different die may be required for each product, or a means of adjusting the die geometry may be required, such as tailoring the height of the slot by applying an adjustable mechanical loading. Major disadvantages of an extrusion die are its mechanical complexity, tight fabrication tolerances, and resulting high cost.
FIG. 1a of the drawings is a weir for forming a curtain according to prior art (Method and Apparatus for Flow Coating Objects, J. Kinzelman, U.S. Pat. No. 3,205,089 issued Sep. 7, 1965). The weir consists of a single channel. Coating composition is delivered to a conduit running through the channel. The conduit has a series of holes along its length to feed the coating composition to the channel.
FIG. 1b is a die forming a curtain according to prior art (Coating a Heat Curable Liquid Dielectric on a Substrate, Curry, II et al; U.S. Pat. 5,298,288 issued Mar. 29, 1994). Inside the die is at least one distribution cavity connected to the inlet and spanning the width of the curtain. The curtain is extruded from a narrow slot adjoining the distribution cavity and spanning the width of the curtain.
FIG. 1c in the drawings shows another weir for forming a curtain according to prior art (Paint Curtain Machine and Method of Painting, J. H. Coleman, U.S. Pat. No. 4,060,649, Nov. 29, 1977). In this case the weir is simply a large basin.